1. Field of the Invention
The present invention relates to an ultrasonic transmitter for radiating ultrasonic waves, an ultrasonic transceiver for radiating ultrasonic waves and receiving echoes of the radiated ultrasonic waves, and a sounding apparatus including an ultrasonic transceiver for detecting objects using ultrasonic waves.
2. Description of the Related Art
Today, sonar apparatuses, such as scanning sonars, are widely used for detecting underwater objects (targets). A scanning sonar for detecting underwater objects in all surrounding directions has a generally cylindrical transducer. The scanning sonar forms an ultrasonic transmitting beam oriented in all directions around the transducer by activating vibrating elements arranged on a cylindrical surface of the transducer. Also, the scanning sonar forms a receiving beam oriented in a particular horizontal direction using a specific number of vertically arranged sets, or columns, of vibrating elements centered on that horizontal direction. Typically, this receiving beam is rotated around the transducer to detect underwater objects in a full-circle area by successively switching the columns of vibrating elements.
It is occasionally necessary for ultrasonic transceivers of scanning sonars of the aforementioned type to control the output power, or amplitude, of ultrasonic waves to prevent mutual interference of the ultrasonic waves between nearby ships which are equipped with the scanning sonars. If the scanning sonar is of a type that radiates ultrasonic waves from a transducer having a plurality of vibrating elements, arranged on the surface of the transducer as stated above, it is also necessary to suppress side lobes of the transmitting beam formed by the transducer (or a combination of the arranged vibrating elements). To achieve this, the ultrasonic transceiver should precisely control driving signals fed into the vibrating elements to control the amplitude of ultrasonic signals radiated from the individual vibrating elements.
Generally, a power amplifier of a transmitter section of the ultrasonic transceiver employs bridge circuits in a final stage, each of the bridge circuits including a plurality of switching devices. The ultrasonic transceiver generates pulse signals for driving the individual vibrating elements by alternately activating the switching devices of the bridge circuits to. The pulse signals cause the vibrating elements to oscillate and thereby radiate ultrasonic signals from the transducer as described in Japanese Patent Application No. 2001-401798, for example.
Two types of bridge circuits are conventionally used in sonar apparatuses. These include a full-bridge circuit using four switching devices and a half-bridge circuit using two switching devices.
FIG. 6 is a general circuit diagram showing an example of a full-bridge circuit, in which field effect transistors (FETs) are used as switching devices.
As shown in FIG. 6, the full-bridge circuit includes a series circuit made up of two switching devices FET11, FET12 and another series circuit made up of two switching devices FET21, FET22, the two series circuits being connected parallel to each other between a driving voltage VB fed from a power supply and ground potential, associated with a vibrating element XD of which both terminals are connected between a joint of the two switching devices FET11, FET12 and a joint of the two switching devices FET21, FET22.
ON/OFF states of the switching devices FET11, FET12, FET21, FET22 are controlled by entering drive signals shown in FIGS. 7A to 7D into the individual switching devices FET11, FET12, FET21, FET22. As a result, the full-bridge circuit supplies a pulse signal shown in FIG. 7E, thereby causing the vibrating element XD to oscillate at its natural resonant frequency. More specifically, when the switching devices FET11, FET21 supply a pulse to the vibrating element XD, the vibrating element XD is caused to resonate and produce free vibration. The vibrating element XD maintains this free vibration even when no input pulse is supplied. Pulses are successively supplied to the vibrating element XD in a controlled fashion to synchronize ON/OFF timings of the switching devices FET11, FET22 and the switching devices FET12, FET21 such that the switching devices FET11, FET22 and the switching devices FET12, FET21 are alternately turned to the ON state with a delay time corresponding to half the period of oscillation of the vibrating element XD (or the reciprocal of the natural resonant frequency of the vibrating element XD). Driven in this way, the vibrating element XD continuously vibrates and radiates an ultrasonic signal having a specific amplitude, in which the amount of attenuation of free vibration is compensated for by the successively input pulses. The amplitude of the ultrasonic signal radiated from the vibrating element XD is controlled by regulating the pulselength of the pulse signal so that the vibrating element XD radiates the ultrasonic signal of a desired amplitude.
In the full-bridge circuit thus configured, a closed loop formed by the vibrating element XD and the switching devices FET12, FET21 has an extremely large impedance if the switching devices FET12, FET21 are in the OFF state when no driving voltage is supplied to the vibrating element XD from the switching devices FET11, FET21, or when the switching devices FET11, FET21 are in the OFF state. When increasing the amplitude of the ultrasonic signal radiated from the vibrating element XD, the pulselength of the pulse signal fed into the vibrating element XD should be increased. If the pulselength of the pulse signal is increased for this reason, periods during which all of the switching devices FET11, FET12, FET21, FET22 are set to the OFF state (i.e., durations α and β shown in FIGS. 7A-7E) are shortened so that the amount of attenuation of vibration of the vibrating element XD becomes relatively small. When decreasing the amplitude of the ultrasonic signal radiated from the vibrating element XD, on the contrary, the pulselength of the pulse signal fed into the vibrating element XD should be decreased. If the pulselength of the pulse signal is decreased for this reason, periods during which all of the switching devices FET11, FET12, FET21, FET22 are set to the OFF state become longer. In this case, time durations during which the closed loop formed by the full-bridge circuit and the vibrating element XD exhibits an extremely large impedance lengthen and the free vibration of the vibrating element XD is limited. Consequently, the vibration of the vibrating element XD is extremely attenuated and, when the vibrating element XD continuously radiates ultrasonic waves at a decreased amplitude, power loss increases resulting in deterioration of efficiency. If the vibrating element XD stops to vibrate before a succeeding driving pulse is fed into the vibrating element XD due to an extremely large amount of attenuation of vibration, the vibrating element XD would no longer be able to continuously transmit the ultrasonic signal.
The aforementioned problem could also occur in conventional half-bridge circuits. FIG. 8 is a general circuit diagram showing an example of a conventional half-bridge circuit.
As shown in FIG. 8, the half-bridge circuit is configured by a pair of series-connected switching devices FET1, FET2 which are connected between a power source supplying a positive driving voltage VB and a power source supplying a negative driving voltage −VB, and a switching device FET3 connected parallel to a vibrating element XD of which one terminal is connected to a joint of the two switching devices FET1, FET2.
As drive signals shown in FIGS. 9A and 9B are input into the individual switching devices FET1, FET2, the aforementioned half-bridge circuit causes the vibrating element XD to vibrate at its natural resonant frequency. In an ordinary half-bridge circuit in which the switching device FET3 is always OFF, or a half-bridge circuit including two switching devices FET1, FET2 and not any switching device FET3, there occurs periods of time when both of the switching devices FET1, FET2 are OFF. Particularly when the pulselength of ultrasonic pulses is reduced to lower the amplitude of the ultrasonic signal emitted from the vibrating element XD, periods during which both of the switching devices FET1, FET2 are set to the OFF state (i.e., durations γ and δ shown in FIGS. 9A-9D) lengthen, so that time durations during which a closed loop formed by the vibrating element XD and the half-bridge circuit exhibits an extremely large impedance lengthen and the free vibration of the vibrating element XD is limited. Consequently, as is the case with the aforementioned full-bridge circuit, the vibration of the vibrating element XD is extremely attenuated and, when the vibrating element XD continuously radiates ultrasonic waves at a decreased amplitude, power loss increases resulting in deterioration of efficiency.
To avoid this inconvenience of the ordinary half-bridge circuit, the conventional half-bridge circuit of FIG. 8 is provided with the switching device FET3 connected parallel to a load (the vibrating element XD). As will be recognized from the circuit diagram of FIG. 8, the half-bridge circuit is controlled in such a manner that the switching device FET3 becomes ON when both of the switching devices FET1, FET2 are in the OFF state. As a result, the impedance of a closed loop formed by the vibrating element XD, the switching device FET3 and the half-bridge circuit is lowered so that the vibrating element XD can maintain free vibration with reduced loss and continuously transmit the ultrasonic signal.
The conventional half-bridge circuit thus configured, however, still has a problem in that the provision of the switching device FET3 connected parallel to the vibrating element XD results in a complex circuit configuration.